So, all headphones play through only one side when connected to your phone. Yeah, try as abil suggested and see if you get stereo sound.

One thing that might help get an idea of the source of the problem is to move the headphone plug while sound is playing. Rotate it, remove and re-insert it. See if, at any point, sound comes through both sides for even a second.
You may have a bad connection at the jack; perhaps a loose wire inside the N900.

Then if you try the same headphones your using in the 900 on another device and they work then you have a hardware problem so take it to Nokia care.

Can you confirm you are not deaf in one ear.

i aint deaf in one ear ....

LOL your crazy read his posts lol.

That made me laugh lol.

Just to be certain, can you read his post again with a finger in your left ear?

Come on now guys lol be serious.
I am not going to even suggest its a software fault because if your not able to identify this problem i would not dare to suggest you reset your N900 lol.

My vote is for hardware issue. Have a tech look at it.

If he is getting any sound from the both sides if it be mono or stereo and "wiggling" the jack does nothing then i have to say the chances of it being software just magnified 99% lol.